1. Field of the Invention
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to, an intermediate transfer device and an image forming apparatus using the intermediate transfer device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member in a process known as intermediate transfer; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Generally, in known color-image forming apparatuses, four image bearing members (which may, for example, be photosensitive drums), one for each of the colors black, yellow, magenta, and cyan, are arranged in tandem facing a belt-type intermediate transfer member (hereinafter referred to as simply “intermediate transfer belt”), and multiple toner images of a respective single color are formed on the image bearing members. Then, the toner images are transferred onto the intermediate transfer belt so that they are superimposed one atop the other, thereby forming a composite toner image on the intermediate transfer belt. This process is known as a “primary transfer process”.
In an image forming apparatus using the intermediate transfer process, the composite toner image on the intermediate transfer belt is transferred onto a recording medium such as a sheet of paper at a secondary transfer nip at which the intermediate transfer belt contacts a secondary transfer member (which may, for example, be a secondary transfer roller) in a process known as a “secondary transfer process”. The intermediate transfer belt is formed into a loop and entrained around a plurality of rollers, one of which faces and presses against the secondary transfer member via the intermediate transfer belt, thereby forming the secondary transfer nip. This roller is known as a secondary transfer counter roller.
In such an image forming apparatus in which an image is formed on the recording medium through the first and the secondary transfer processes, transferability is decreased due to environmental changes. In view of the above, various approaches have been proposed in an attempt to solve the problem.
For example, in one approach, the secondary transfer roller is grounded, and a certain amount of electrical current having the same polarity as that of the toner is supplied to the secondary transfer counter roller disposed inside the loop formed by the intermediate transfer belt. In this configuration, even when an electrical resistance of the devices such as the intermediate transfer belt, the secondary transfer counter roller, and the secondary transfer roller fluctuates, degradation of the transferability in the secondary transfer process is prevented.
Although effective, there is a drawback to the above-described approach. That is, when the toner image on the intermediate transfer belt bears a large amount toner, when an amount of electric charge or non-electrostatic adhesion of toner of the toner image is large, and/or a recording medium has a high volume resistance and includes insulation properties such as an OHP sheet or the like, a secondary transfer voltage for supplying a certain amount of secondary transfer current to the secondary transfer counter roller becomes large. The large secondary transfer voltage requires a high-voltage power supply capable of supplying a high voltage and a high-voltage wiring harness with a high withstand voltage, which are generally expensive. Furthermore, an insulating member with a high withstand voltage, which is also generally expensive, needs to be provided around an electrode connected to the secondary transfer counter roller, and a large creepage distance and clearance are required relative to the secondary transfer counter roller.
Moreover, with recording media having different surface characteristics, it is difficult to optimally transfer the toner image onto the recording medium under the same transfer conditions. Accordingly, in order to optimally transfer the toner image onto a recording medium having surface characteristics different from the preceding recording medium, for example, a bias of the same polarity as the toner is applied to the secondary transfer counter roller with a constant current or a constant voltage while applying a bias of the opposite polarity to the polarity of toner with a constant current or a constant voltage, to the secondary transfer roller. In this configuration, application of the bias to the secondary transfer counter roller and application of the bias to the secondary transfer roller are performed selectively in accordance with transfer conditions, and the applied voltages are relatively high. Such high-voltage power supplies capable of supplying a high voltage are relatively large and expensive, thus hindering efforts to provide the low-cost, compact image forming apparatuses for which there is market demand.
In order to enhance transferability when forming an image on a recording medium having a coarse surface such as an embossed sheet, the surface of the recording medium on which the toner image is transferred is charged to the opposite polarity to the polarity of the toner prior to the transfer process. In this configuration, upon transfer of the toner image, a transfer bias consisting of an alternating current (AC) voltage superimposed on a direct current (DC) voltage is supplied to the secondary transfer roller, and the rear surface of the intermediate transfer belt is grounded. Although effective, the transferability of secondary transfer is still degraded when the electrical resistance of the devices and/or the recording medium fluctuates due to environmental changes.
In order to prevent degradation of transferability in the secondary transfer process in a high-speed image forming apparatus when environmental changes cause the electrical resistance of the devices and the recording medium to fluctuate, a bias of the same polarity as the toner can be applied to the intermediate transfer belt by the secondary transfer counter roller during the secondary transfer process to keep the electrical current constant while applying a bias having the opposite polarity to the polarity of the toner to the secondary transfer roller to keep the voltage constant.
In this configuration, a direct-current power supply is employed. When using a recording medium having a coarse surface such as the embossed sheet, a transfer potential of recessed portions of the recording medium is lower than that of projecting portions, so that the toner is not transferred adequately to the recessed portion, thereby yielding a resulting image with white spots.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining good transferability regardless of fluctuation of the electrical resistance of devices as well as surface conditions of recording media.